* Astronomy

Title: Swift-UVOT captures the earliest ultraviolet spectrum of a Gamma Ray Burst
Authors: N.P.M. Kuin, W. Landsman, M.J. Page, P. Schady, M. Still, A.A. Breeveld, M. De Pasquale, P.J. Brown, M.Carter, C. James, P.A. Curran, A. Cucciara, C. Gronwall, S.T. Holland, E. Hoversten, S. Hunsberger, T. Kennedy, S. Koch, H. Lamoureux, F.E. Marshall, S.R. Oates, A. Parsons, D. Palmer, P. Roming, P.J. Smith

We present the earliest ever ultraviolet spectrum of a gamma-ray burst (GRB) as observed with the Swift-UVOT. The spectrum of GRB 081203A was observed for 50 seconds with the UV grism starting 251 seconds after the Swift-BAT trigger when the GRB was of u ~13.4 mag and still rising to its peak optical brightness. The UV grism spectrum shows a damped Ly-alpha line, Ly-beta, and the Lyman continuum break at a redshift z = 2.05 ±0.01. A model fit to the Lyman absorption implies log N(HI) = 22.0 ±0.2 cm-2, which is typical for GRB host galaxies with damped Ly-alpha absorbers. This observation of GRB 081203A demonstrates that for GRBs brighter than v ~14 mag and with 0.5 < z < 3.5 the UVOT will be able to provide redshifts, and probe for damped Ly-alpha absorbers within 4-6 minutes from the time of the Swift-BAT trigger.

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